1. Field of the Invention
The present invention generally relates to a thermo-shrinkable polyester film, and specifically, to a thermo-shrinkable polyester film suitable for a labeling film or a covering of a container having a complicated shape such as, for example, cosmetic containers, fragrance or toiletry containers, or liquid medication containers.
2. Description of the Related Art
Conventionally, thermo-shrinkable films used as labels or coverings provided on bottles are mainly formed of poly(vinyl chloride), polystyrene, polyethylene and the like. Recently, thermo-shrinkable polyester films have been a focus due to the following problems with the above-mentioned materials. Poly(vinyl chloride) generates chlorine gas when burned for disposal. Polyethylene is difficult for use in printing characters and graphics on a film formed thereof. Moreover, when these materials are used for labels around polyethylene terephthalate (PET) bottles, the material of the labels needs to be distinguished from the material of the PET bottles in order to recycle the PET bottles.
Conventional thermo-shrinkable polyester films are not satisfactory as labeling films. The films shrink rapidly resulting in wrinkles, non-uniform shrinkage, and distortion. Further, the post-shrinkage films are easily ruptured by an external impact.
In order to solve these problems, Japanese Publication for Opposition No. 7-77757 discloses a method for significantly reducing the elongation at rupture in the direction perpendicular to a main shrinking direction of the film to improve the post-shrinkage state of the film.
Japanese Laid-Open Publication No. 58-64958 discloses a method for reducing the shrinking force of the film to improve the post-shrinkage state of the film.
However, the methods disclosed in these documents do not provide a satisfactory post-shrinkage state of the film when applied to produce labels for small PET bottles. Such bottles' labels are shrunk by passing the bottles through a shrinkage tunnel for a relatively short time.
Furthermore, demand for the recycling of bottles formed from thermoplastic polymers has increased greatly in order to deal with the environmental problems. Especially, the recycling of PET bottles is of great concern, and it is imperative to urgently establish a recycling system therefor. Labels provided around PET bottles generally include, for example, stretched labels formed of polyolefin and the like, thermo-shrinkable labels formed of polyester, polystyrene, vinyl chloride and the like, and tack labels formed of polypropylene and the like. The PET bottles are typically recycled as follows. The bottles are collected from consumers with the labels thereon and eventually brought to recycling companies or plants. The bottles are washed and then treated by primary pulverization to remove the labels. The substance obtained by the pulverization still includes a great amount of the label material. Accordingly, the substance is further treated by secondary pulverization and separation of the label material using specific gravity in liquids, de-watering and drying, separation of the label materials using wind, and pelletization. Thus, recycled pellets are obtained.
Recently, in order to improve the label removal efficiency, the labels are detached from the bottles after the bottles are washed but before pulverization and pelletization.
For performing such a process, labels having perforations are used to facilitate the detachment of the labels. The labels are detached from the bottles by tearing the labels along the perforations.
Films obtained by the methods disclosed in the above-mentioned documents are inferior in the ease of tearing along perforations after thermally shrunk and thus are not satisfactory for a shrinkable label film.
Conventional thermo-shrinkable polyester films which are shrinkable in one direction have further problems in that wrinkles, non-uniform shrinkage and insufficient shrinkage, occur when subjected to shrinkage to cover containers having complicated shapes such as, cosmetic containers, fragrance and toiletry containers, or liquid medication containers.
In order to solve such a problem, Japanese Laid-Open Publication No. 3-29763 discloses a thermo-shrinkable polyester film which is shrinkable in a direction perpendicular to the main shrinking direction. Japanese Laid-Open Publication No. 5-185510 discloses a method for restricting the drawing ratio in both the longitudinal and transverse directions of the film so that the shrinkage ratio in the direction perpendicular to the main shrinking direction is about 15%.
However, films obtained by the methods disclosed in these documents possess very poor sealing strengths (described below in the Evaluation section (9)), the sealing strengths being required for covering the above-mentioned complex-shaped containers.
In order to overcome the problem, Japanese Patent No. 2,566,568 discloses a thermo-shrinkable polyester film having an improved heat seal property.
However, films obtained by the method disclosed in the above-mentioned patent do not possess a satisfactory sealing strength and also have insufficient shrinkage ratios in the direction perpendicular to the main shrinking direction. Such films undesirably tend to cause wrinkles, non-uniform shrinkage and distortion, and thus are not practically usable.